Method for reducing sheeting and agglomerates during olefin polymerisation

ABSTRACT

The present invention relates to a method for reducing/suppressing sheeting or agglomerates during polymerisation of olefins, especially during the fluidised bed gas phase polymerisation of olefins. In particular, the present invention relates to a method for reducing/suppressing sheeting or agglomerates during the product grade transition and/or catalyst transitions occuring polymerisation of olefins.

[0001] The present invention relates to a method forreducing/suppressing sheeting or agglomerates during polymerisation ofolefins, especially during the fluidised bed gas phase polymerisation ofolefins. In particular, the present invention relates to a method forreducing/suppressing sheeting or agglomerates during the product gradetransition and/or catalyst transitions occurring during polymerisationof olefins.

[0002] Processes for the co-polymerisation of olefins in the gas phaseare well known in the art. Such processes can be conducted for exampleby introducing the gaseous monomer and comonomer into a stirred and/orgas fluidised bed comprising polyolefin and a catalyst for thepolymerisation.

[0003] In the gas fluidised bed polymerisation of olefins, thepolymerisation is conducted in a fluidised bed reactor wherein a bed ofpolymer particles is maintained in a fluidised state by means of anascending gas stream comprising the gaseous reaction monomer. Thestart-up of such a polymerisation generally employs a bed of polymerparticles similar to the polymer which it is desired to manufacture.During the course of polymerisation, fresh polymer is generated by thecatalytic polymerisation of the monomer, and polymer product iswithdrawn to maintain the bed at more or less constant volume. Anindustrially favoured process employs a fluidisation grid to distributethe fluidising gas to the bed, and to act as a support for the bed whenthe supply of gas is cut off. The polymer produced is generallywithdrawn from the reactor via a discharge conduit arranged in the lowerportion of the reactor, near the fluidisation grid. The fluidised bedconsists in a bed of growing polymer particles. This bed is maintainedin a fluidised condition by the continuous upward flow from the base ofthe reactor of a fluidising gas.

[0004] The polymerisation of olefins is an exothermic reaction and it istherefore necessary to provide means to cool the bed to remove the heatof polymerisation. In the absence of such cooling the bed would increasein temperature and, for example, the catalyst becomes inactive or thebed commences to fuse. In the fluidised bed polymerisation of olefins,the preferred method for removing the heat of polymerisation is bysupplying to the polymerisation reactor a gas, the fluidising gas, whichis at a temperature lower than the desired polymerisation temperature,passing the gas through the fluidised bed to conduct away the heat ofpolymerisation, removing the gas from the reactor and cooling it bypassage through an external heat exchanger, and recycling it to the bed.The temperature of the recycle gas can be adjusted in the heat exchangerto maintain the fluidised bed at the desired polymerisation temperature.In this method of polymerising alpha olefins, the recycle gas generallycomprises the monomer and comonomer olefins, optionally together with,for example, an inert diluent gas such as nitrogen or a gaseous chaintransfer agent such as hydrogen. Thus, the recycle gas serves to supplythe monomer to the bed, to fluidise the bed, and to maintain the bed atthe desired temperature. Monomers consumed by the polymerisationreaction are normally replaced by adding make up gas or liquid to thepolymerisation zone or reaction loop.

[0005] A gas fluidised bed polymerisation reactor is typicallycontrolled to achieve a desired melt index and density for the polymerat an optimum production. Conditions within the polymerisation reactorhave to be carefully controlled to reduce the risk of agglomerate and/orsheet formation which may ultimately lead to bed instabilities and aneed to terminate the reaction and shut down the reactor. This is thereason why commercial scale reactors are designed to operate well withinproven stable operating zones and why the reactors are used in acarefully circumscribed fashion.

[0006] Even within the constraints of conventional, safe operation,control is complex adding further difficulty and uncertainty if onewishes to find new and improved operating conditions.

[0007] There is no generally accepted view as to what causesagglomerates or sheeting. Agglomerates or sheets can, for example, formwhen the polymerisation temperature is too close to the polymersintering temperature or when the polymer particles become excessivelysticky. Highly active fine particles can, for example, concentrate inthe upper elevations of the polymerisation zone, towards the top of thefluidised bed and in the powder disengagement zone above the bed thusleading to local hot spots and potential agglomeration and/or sheeting.

[0008] According to the present invention a thorough understanding ofsheeting and agglomeration mechanisms has allowed us to develop productspecific operating windows where sheeting or agglomeration do not occur.This is illustrated with comparative examples, that the newly developedoperating windows are unusual and that the “man skilled in the art”would previously have avoided such operation for fear of encounteringthe very operating problems that the technique overcomes.

[0009] An embodiment of the present invention finds its source in thestudy of the properties of reacting polymer particles. It has been foundthat sheeting or agglomeration do not occur when instantaneous particleproperties (mechanical, physical, dielectric . . . ) are maintained in abounded window.

[0010] Industrial operation usually requires the production of differentgrades. Product transition usually corresponds to a variation inparticle properties. It is an embodiment of the present invention topropose a procedure to limit the change of critical particle propertiesduring grade transitions. This is performed by continuously changingoperating conditions such that particle properties remain in a boundedwindow during grade transition.

[0011] Agglomerates or sheeting are responsible for costly productionlosses, unreliable operation, strong limitations on plant performanceand considerable damage to the global polyolefin businesses.

[0012] The present invention allows us to increase plant capacity by upto 50% for certain grades when the limitation is sheeting oragglomerates.

[0013] The shape of agglomerates or sheeting varies widely in size andaspect but they are usually similar in most respects. One of the mostprobable cause of agglomeration or sheeting (when operating far frompowder sintering temperature) is the accumulation of powder at thereactor walls. We believe that the layer formed at the wall can be asthin as a few micrometers and up to several centimetres. Thecorresponding sheet or agglomerates have comparable thickness. Thelength of agglomerates can vary between a few centimetres and severalmeters.

[0014] A visual inspection at the outlet of the reactor can be used inorder to monitor the presence of sheets or agglomerates. Temperatureprobes can also monitor the formation of the sheets or agglomerates. Theprobes can be anywhere between the insulation of the reactor (when used)to the centre of the reactor. The analysis of temperature probes isbelieved to be an excellent indication of the formation of sheets oragglomerates. A surprising lowering of the temperature at the wallindicates that particles adhere, causing a probable insulating effectfrom the bulk temperature. Deviations of a few degrees up to more than20° C. (sometimes 35° C.) have been commonly observed. When skintemperatures start to rise, it indicates the presence of a reactinglayer of powder at the wall. The corresponding zone being of limitedheat transfer, such cases often lead to an agglomerate storm. Anothervery advantageous monitoring tool consists in optical fibres located onthe surface of the reactor; examples thereof can be found in Frenchpatent application n^(o)0007196 filed on Jun. 6, 2000 by BP ChemicalsSNC.

[0015] It is also believed that the layer of powder at the wall (fusedor not) may be able to fall into the reactor. This is observed by aclear disruption of fluidisation patterns (pressure probes).

[0016] When sheeting or agglomeration occurs, industrial experience (andthe theory) has taught us to reduce operating temperature untilagglomeration stops. This procedure is basic and is usually used byoperators. However, it does not solve the root of the problem andagglomerates can reappear later, especially during grade transition. Thelow temperature operation is also detrimental regarding heat exchangelimitations.

[0017] More than 20 years of publications indicate that electrostaticityin the bed is the contributing factor to agglomeration at the wall.However, an analysis of the prior art methods disclosed in theliterature tend to prove that a plant control based on electrostaticmeasurement is not industrially satisfactory since the electrostaticmeasurement tool per se is influenced by too many factors which aretotally not representative of fouling problems.

[0018] In this respect, the present invention indicates that the problemof agglomeration or sheeting can be solved regardless of staticelectricity considerations.

[0019] The production losses, down time, reactor cleaning and otherproblems related to sheeting or agglomeration are contributory to a highproportion of unplanned reactor downtime. Therefore, there is anon-going need to provide additional methods of agglomeration/sheetingcontrol.

[0020] Accordingly, the present invention provides a process forreducing/suppressing sheeting or agglomerates during polymerisation ofolefins, especially during the fluidised bed gas phase polymerisation ofolefins. In particular, the present invention relates to a method forreducing/suppressing sheeting or agglomerates during start-up,transitioning and steady state olefin polymerisation.

[0021] This paragraph summarises the approach used to define the optimumoperating window for polymer particle properties according to thepresent invention.

[0022] The Applicants have found that numerous grade transitions andstart-up procedures in industrial operation are characterised by drasticchanges in instantaneous particle properties which lead to agglomeratesand/or sheeting at the reactor wall.

[0023] A stochastic model of the fluidised bed based on a refinedMonte-Carlo approach has been built in order to help understandpotential agglomeration mechanisms.

[0024] The behaviour of a representative set of 10 million particles issimulated in order to evaluate the amount of overheating particles, i.e.those particles for which the surface temperature is higher thansintering temperature, i.e. the temperature which is slightly inferiorbelow the melting temperature and which is representative of thetemperature at which the polymer powder starts to agglomerate. For thepurpose of the present description and appended claims, the sinteringtemperature of the polymer powder under reactor operating conditions isthe temperature at which a bed of said polymer powder in contact with agas having the same composition as the reactor recycle gas used inproducing the polymer powder will sinter and form agglomerates whenfluidization velocity is at maximum taking into account the fineparticle entrainment limitation. The sintering temperature is decreasedby decreasing the resin density, by increasing the melt index and byincreasing the amount of dissolved monomers.

[0025] The particle temperature is estimated by solving heat transferequations at the level of the particle. The fundamental mechanismsinvolved in that process can be divided in 2 categories: mechanismsresponsible for heat generation (polymerisation reaction depending onwell quantified kinetics) and equations governing heat transfer.

[0026] Heat generation is well quantified based on well known reactionkinetics and the stochastic approach allows us to describe thecomplexity of the fluid bed reactor using statistical dispersion of keyparameters (such as partial pressure of reactants, initial concentrationof active sites, level of impurities, . . . ) around their quasi-steadystate average values. This process allowed us to generate arepresentative set of reacting particles in the reactor (10⁷).

[0027] Heat transfer quantification is more complex to quantify due tocompetition between the different mechanisms involved: for eachparticle, heat transfer is quantified by considering local gas velocityat the level of the particle (governed mainly by particle size andposition in the reactor), vaporisation of liquid at the surface of theparticle (in liquid injection mode, e.g. condensation mode) and gascomposition, pressure and temperature. As for heat generation, astochastic approach is used to simulate the fluidised bed behaviour.

[0028]FIG. 2 illustrates the typical results obtained for gas phasepolymerisation process wherein the mass percentage of overheatingparticles is given for increasing polymerisation temperature.

[0029] At very high polymerisation temperature, operation is too closeto the sintering temperature of the powder and particles massivelyagglomerate. Operation is highly unstable and the risk of agglomeratingthe entire bed is high. Operators are constantly aware of this dangerand keep operation far away from the powder sintering limit, i.e. in the“common operating window”.

[0030] However, the typical Overheating/Temperature curve also indicatesthat agglomerates can be formed at lower temperature and points out theexistence of a local minimum where temperature is still high but therisk of agglomeration or sheeting is very low.

[0031] The corresponding operating window is the optimumagglomerate/sheeting-free operating window (as indicated on the righthand side of FIG. 2), which is also called the high temperature optimumoperating window.

[0032] In fact, operators being aware of the risk of agglomeration athigh temperatures prefer to operate with a significant safety margin atmuch lower temperatures than the sintering temperature. It is clear thatthere is a resistance in the art to increasing operating temperaturethrough a fear of encountering powder sintering limits. However, thepresent invention demonstrates that it is possible by acting againstthis natural tendency, i.e. by increasing the operating temperature, tocontrol advantageously the polymerisation while reducing and/oreliminating the agglomeration/sheeting risks.

[0033] It is therefore an object of the present invention to provide aprocess for reducing/suppressing sheeting or agglomerates duringpolymerisation of olefins, characterised in that the operatingtemperature is controlled in order to maintain the polymer particle inits high temperature optimum operating window throughout thepolymerisation.

[0034] Indeed, once the man skilled in the art is aware of the existenceof said optimum operating window, he is able to control his plant, andin particular the operating temperature, in such a way that the polymerparticles remain in said optimum operating window.

[0035] This process is preferably applied during the fluidised bed gasphase polymerisation of olefins, especially during start-up andtransition, more preferably during product grade transition.

[0036] While not wishing to be bound by the theory, the explanation forthe existence of an increasing risk of sheeting/agglomeration at lowtemperature is related to instantaneous reacting particle properties.Indeed, temperature highly affects instantaneous particle properties(mechanical, physical and dielectric). When particle temperature isdecreased (this can be done by decreasing polymerisation temperature),particles become more brittle, and surface properties are modified.

[0037] At low temperature, the generation of fines and micro-finesdrastically increases. Although the fines fraction represents a lowpercentage in mass, it represents a considerable number of particleswhich are susceptible to adhere to the reactor wall due to their smallsize.

[0038] Conversely, when operating temperature is controlled in order toremain in the high temperature window throughout the polymerisation, ithas been unexpectedly found that (micro-)fines generation could belowered at a level where the presence of said (micro-)fines did notentrain any irreversible agglomeration phenomenon.

[0039] The stochastic model for the fluidised bed also pointed out thesimple fact that agglomerates or sheeting are formed when heat exchangeis limited. When this is the case, the fraction of overheating particlesis highly dependent on operating parameters such as condensation rate,fluidisation velocity, polymerisation rate (heat generated) andprepolymer or catalyst fines. On the contrary, when operating in theoptimum window for particle properties, heat exchange is not limitingand the operating conditions previously mentioned do not affect thefraction of overheating particles to the same extent. In that case,plant performance can be increased by pushing catalyst productivity andproduction rate. (FIG. 3)

[0040] The last observation to be mentioned concerns the most commonlyused operating window which is the so called “low temperature window”(left hand of FIG. 2). It corresponds to the case where operatingtemperature is sufficiently low so powder does accumulate at the wallbut particle overheating remains controllable. This operating window canbe considered as metastable. Although it is the commonly used operatingwindow, we have found that it is non-optimised in many respects: heatexchange capacity is limited, agglomerates or sheeting can form whenoperating conditions are changed or production rate is increased, andmost likely during grade transitions when particle properties aresignificantly changed.

[0041] Another object of the present invention, which is illustrated indetail in the following examples, relates to a process forreducing/suppressing sheeting or agglomerates during transition betweentwo different polymer products made during the polymerisation ofolefins, characterised in that the operating temperature is controlledin order to maintain the polymer particle in its high temperatureoptimum operating window throughout the transition.

[0042] In order to further analyse the corresponding phenomena, acriteria has been used to follow the changes in particle properties inreal time, i.e. the instantaneous particle properties.

[0043] The particle properties regarded as important are the following:toughness, brittleness, crystallinity, conductivity, softeningtemperature, and sintering temperature.

[0044] Amongst the different possibilities, a combined criteria has beenselected for the following reasons:

[0045] It varies with polymer crystallinity

[0046] It is a marker of polymer dielectric properties

[0047] It is derived from a mechanical property (Tensile Strength).

[0048] The general form of the criteria is the following:

Crit=ƒ(Property Model 1, Property Model 2, . . . )

[0049] Structure/Property Models are used to predict resin propertiesin-real time in order to build an on-line criteria for monitoringagglomerate/sheeting-free operating windows.

[0050] Resin properties are predicted from resin molecular structurewhich is relatively simple in the case of simple polymers such aspolyethylene or polypropylene.

[0051] In the following description, the examples of Linear Low DensityPolyethylene (LLDPE) and High Density Polyethylene (HDPE) will becovered. However, it is clear that the generality of the definedcriteria is applicable to a large range of applications.

[0052] Molecular Structure for LLDPE/HDPE:

[0053] The simple molecular structure in that case can be described bythe average polymer chain length, the dispersion of the chain lengths(polydispersity), the type of short chain branching (type of comonomer),the amount of short chain branching, the short chain branchingdistribution, and the size and amount of long chain branching.

[0054] In practice, all of this information is not necessary to predictresin properties to sufficient accuracy when the range of products beingconsidered is limited (e.g. to certain catalyst types or even specificcomonomers). A limited set of relevant parameters have in thesecircumstances been found to be highly sufficient. Indeed, from a Processmonitoring point of view, the simplest description of resin molecularstructure is highly desirable: for a given catalyst and comonomer typethe first order parameters to be considered are the average polymerchain length and the amount of comonomer. Consequently, the simplestapproach is to use Melt-Index (average molecular weight) and Density(amount of comonomer) to describe the changes in resin molecularstructure. The Criteria “Crit” will depend on the specifics of thedifferent comonomer types and catalysts.

[0055] The main difficulty in the Structure/Properties approach is theprediction of particle properties in reacting conditions.

[0056] This problem has been solved from a process monitoring point ofview by quantifying the effects of the most sensitive parameters only.These are the parameters having a great influence on particle propertiesin the usual range of variation in industrial operation. For instance,the criteria “Crit” will be modified in order to differentiate plantoperating at high polymerisation rates. Indeed, a high polymerisationrate will affect particle properties via the particle temperature whichis an important parameter for particle properties. However, the effectof this parameter being of second order, it is not mandatory toincorporate it in a more detailed model.

[0057] Example of Structure/Property Model: Particle Tensile Strength

[0058] In this example, a so called “Particle Tensile Strength” propertyis predicted from resin molecular structure (Melt-Index and Density inthat case). It is an extrapolation of Resin Tensile Strength atpolymerisation temperature.

[0059] The model has been built from measurements of Tensile Strengthperformed on injection moulded samples (ASTM n^(o) D638-89). Over 150samples have been tested covering a wide range of densities andMelt-Indexes. The comparison between predictions and measurements isgiven in FIG. 1 for RIGIDEX™ product types.

[0060] Such models being available, particle properties in the reactorcan be monitored on-line via the prediction of Melt-Index and Density inreal time. We should take the opportunity to mention here that thecritical particle properties involved in the agglomeration mechanismsare the so called “instantaneous properties” which correspond to theproperties of the resin formed instantaneously in the reactingconditions at a given time. The “instantaneous properties” are differentfrom the pellet properties which correspond to a mixture of differentresins formed continuously in the fluidised bed (averaging effect). The“instantaneous properties” require the use of accurate process modelsable to predict powder properties from operating parameters.

[0061] By taking into consideration the above, another embodiment of thepresent invention is to provide an effective process forreducing/suppressing sheeting or agglomerates during polymerisation ofolefins, process characterised in that the above criteria “Crit” ismaintained in a bounded window which corresponds to the high temperatureoptimised operating window.

[0062] Thus, the optimum operating window can be reached by controllinginstantaneous particle properties, preferably mechanical properties,e.g. tensile strength as described hereabove.

[0063] It is therefore an object of the present invention to provide aprocess for reducing/suppressing sheeting or agglomerates duringpolymerisation of olefins, characterised in that the instantaneousproperties of the growing polymer particles formed throughout thepolymerisation are maintained such that there is no irreversibleformation of agglomerates through generation of (micro-)fines.

[0064] It is a further object of the present invention to provide aprocess for reducing/suppressing sheeting or agglomerates duringtransition between two different polymer products made duringpolymerisation of olefins, characterised in that the instantaneousproperties of the growing polymer particles formed throughout thetransition are maintained such that there is no irreversible formationof agglomerates through generation of (micro-)fines.

[0065] Indeed, once the man in the art is aware of the existence of thehigh temperature optimum operating window according to the presentinvention, i.e. where there is no irreversible formation of agglomeratesthrough generation of (micro-)fines, he will take automatically allnecessary steps in order to maintain the instantaneous properties of thegrowing polymer particles in its safe optimised window.

[0066] According to a preferred embodiment of the present invention, andas explained hereabove, the instantaneous properties of the growingpolymer particles are predicted by using a structure/property model.

[0067] According to another preferred embodiment of the presentinvention, the instantaneous properties are mechanical properties of thegrowing polymer.

[0068] According to a further preferred embodiment of the presentinvention, it is the instantaneous tensile strength of the growingpolymer particles which is maintained in its safe optimised window.

[0069] Polymerisation rate and fluidisation velocity may slightlyinfluence these criteria's.

[0070] For example, in the case of the tensile strength propertycriteria, for a fluidised bed polymerisation, when condensation is used,or kinetics are smoother and fluidisation velocity is higher, theoperating window is wider and therefore the optimum operating windowcorresponds to higher values of the criteria.

[0071] At temperature close to the sintering temperature, the criteriadecreases rapidly to take into account the softening of the particle andthe loss of mechanical toughness (and brittleness).

[0072] One of the main advantages according to the present invention isthat the man skilled in the art has now at his disposal a practical toolwhich allows him to determine the optimum operating window, and inparticular the optimum temperature in order to avoid sheeting oragglomerates during the polymerisation of olefins, preferably during thefluidised bed gas phase polymerisation of olefins, in particular duringpolymer product transition.

[0073] In particular, once the man skilled in the art is able to produceone polymer grade in said optimum operating window, i.e. once he is inthe position of fulfilling the above instantaneous particle propertycriteria's, he is also automatically able to proceed efficiently withpolymer grade transitioning by keeping the said criteria at more or lessthe same value through the control of the operating temperature, asdisclosed in the examples.

[0074] It is another embodiment of the present invention to provide foran alternative method for determining the high temperature optimumoperating window of a polymer A having a density A (d_(A)), a melt indexA (MI_(A)) and a sintering temperature T_(SA) which is produced at anoperating temperature A (T_(A)) characterised in the following steps:

[0075] 1. monitor sheet formation

[0076] 2. if sheet are (being) formed, increase the temperature to avalue T_(X) which is equal to or higher than [0.5*(T_(A)+T_(SA))] andlower than the sintering temperature of the formed polymer minus twodegrees centigrade

[0077] 3. if sheets are not (being) formed, the actual polymerisationtemperature becomes part of the high temperature optimum operatingwindow of the polymer A under the existing polymerisation conditions.

[0078] Optionally, just before or just after step 2, if the sheetformation process can not be effectively controlled, proceed with acomplete polymerisation stop process and restart the polymerisation at atemperature which is at least equal to T_(A), preferably at least equalto T_(X).

[0079] It is a further embodiment of the present invention to providefor an alternative method for determining the optimum operating windowof a transition polymer AB during the transition between a polymer A(d_(A), MI_(A), sintering temperature T_(SA), produced under temperatureT_(A)) to a polymer B (d_(B), MI_(B), sintering temperature T_(SB))wherein the said transition polymer AB (d_(AB), MI_(AB)) is being formedcharacterised in the following steps:

[0080] 1. monitor sheet formation

[0081] 2. if d_(B)>d_(A) and MI_(B)≦MI_(A), increase the polymerisationtemperature to a value T_(X1) which is equal to or higher than[0.5*(T_(A)+T_(SB))] and lower than the sintering temperature of theformed polymer minus two degrees centigrade

[0082] 3. if sheet are (being) formed, continue to increase thetemperature to a value T_(X2) higher than [0.5*(T_(X1)+T_(SB))] andlower than the sintering temperature of the formed polymer minus twodegrees centigrade

[0083] 4. if sheets are not (being) formed, the actual polymerisationbecomes part of the high temperature optimum operating window of thetransition polymer AB under the existing polymerisation conditions.

[0084] Optionally, step 3 can be repeated by replacing T_(X1) by T_(X2)in the equation.

[0085] Optionally, just before or just after step 2, if the sheetformation process can not be effectively controlled, proceed with acomplete polymerisation stop process and restart the polymerisation at atemperature which is at least equal to T_(X1).

[0086] Once the above transitioning process has been completed and thed_(B) MI_(B) values of polymer B reached, i.e. when polymer B issuccessfully produced without sheet, then the actual polymerisationtemperature becomes part of the high temperature optimum operatingwindow of the polymer B under the existing polymerisation conditions.

[0087] It is a further embodiment of the present invention to providefor an alternative method for determining the optimum operating windowof a transition polymer AB during the transition between a polymer A(d_(A), MI_(A), sintering temperature T_(SA), which is produced atT_(A)) to a polymer B (d_(B), MI_(B), sintering temperature T_(SB))wherein the said transition polymer AB (d_(AB), MI_(AB)) is being formedcharacterised in the following steps:

[0088] 1. monitor sheet formation

[0089] 2. if d_(B)<d_(A) and MI_(B)≧MI_(A), decrease the polymerisationtemperature to a value T_(Y1) equal to or higher than[T_(SB)−1.2*(T_(SA)−T_(A))] and lower than the sintering temperature ofthe formed polymer minus two degrees centigrade

[0090] 3. if sheet are (being) formed, increase the polymerisationtemperature to a value T_(Y2) equal to or higher than[0.5*(T_(Y1)+T_(SB))] and lower than the sintering temperature of theformed polymer minus two degrees centigrade

[0091] 4. if sheets are not (being) formed, the actual polymerisationtemperature becomes part of the high temperature optimum operatingwindow of the transition polymer AB under the existing polymerisationconditions.

[0092] The process according to the present invention is particularlysuitable for the manufacture of polymers in a continuous gas fluidisedbed process. Illustrative of the polymers which can be produced inaccordance with the invention are the following:

[0093] SBR (polymer of butadiene copolymerised with styrene),

[0094] ABS (polymer of acrylonitrile, butadiene and styrene),

[0095] nitrile (polymer of butadiene copolymerised with acrylonitrile),

[0096] butyl (polymer of isobutylene copolymerised with isoprene),

[0097] EPR (polymer of ethylene with propylene),

[0098] EPDM (polymer of etylene copolymerised with propylene and a dienesuch as hexadiene, dicyclopentadiene or ethylidene norborene),

[0099] copolymer of ethylene and vinyltrimethoxy silane, copolymer ofethylene and one or more of acrylonitrile, maleic acid esters, vinylacetate, acrylic and methacrylic acid esters and the like

[0100] In an advantageous embodiment of this invention, the polymer is apolyolefin preferably copolymers of ethylene and/or propylene and/orbutene. Preferred alpha-olefins used in combination with ethylene and/orpropylene and/or butene in the process of the present invention arethose having from 4 to 8 carbon atoms. However, small quantities ofalpha olefins having more than 8 carbon atoms, for example 9 to 40carbon atoms (e.g. a conjugated diene), can be employed if desired. Thusit is possible to produce copolymers of ethylene and/or propylene and/orbutene with one or more C₄-C₈ alpha-olefins. The preferred alpha-olefinsare but-1-ene, pent-1-ene, hex-1-ene, 4-methylpent-1-ene, oct-1-ene andbutadiene. Examples of higher olefins that can be copolymerised with theprimary ethylene and/or propylene monomer, or as partial replacement forthe C₄-C₈ monomer are dec-1-ene and ethylidene norborene.

[0101] According to a preferred embodiment, the process of the presentinvention preferably applies to the manufacture of polyolefins in thegas phase by the copolymerisation of ethylene with but-1-ene and/orhex-1-ene and/or 4MP-1.

[0102] The process according to the present invention may be used toprepare a wide variety of polymer products for example linear lowdensity polyethylene (LLDPE) based on copolymers of ethylene withbut-1-ene, 4-methylpent-1-ene or hex-1-ene and high density polyethylene(HDPE) which can be for example copolymers of ethylene with a smallportion of higher alpha olefin, for example, but-1-ene, pent-1-ene,hex-1-ene or 4-methylpent-1-ene.

[0103] When liquid condenses out of the recycle gaseous stream, it canbe a condensable monomer, e.g. but-1-ene, hex-1-ene, 4-methylpent-1-eneor octene used as a comonomer, and/or an optional inert condensableliquid, e.g. inert hydrocarbon(s), such as C₄-C₈ alkane(s) orcycloalkane(s), particularly butane, pentane or hexane.

[0104] The process is particularly suitable for polymerising olefins atan absolute pressure of between 0.5 and 6 MPa and at a temperature ofbetween 30° C. and 130° C. For example for LLDPE production thetemperature is suitably in the range 75-110° C. and for HDPE thetemperature is typically 80-125° C. depending on the activity of thecatalyst used and the polymer properties desired.

[0105] The polymerisation is preferably carried out continuously in avertical fluidised bed reactor according to techniques known inthemselves and in equipment such as that described in European patentapplication EP-0 855 411, French Patent No. 2,207,145 or French PatentNo. 2,335,526. The process of the invention is particularly well suitedto industrial-scale reactors of very large size.

[0106] The polymerisation reaction may be carried out in the presence ofa catalyst system of the Ziegler-Natta type, consisting of a solidcatalyst essentially comprising a compound of a transition metal and ofa cocatalyst comprising an organic compound of a metal (i.e. anorganometallic compound, for example an alkylaluminium compound).High-activity catalyst systems have already been known for a number ofyears and are capable of producing large quantities of polymer in arelatively short time, and thus make it possible to avoid a step ofremoving catalyst residues from the polymer. These high-activitycatalyst systems generally comprise a solid catalyst consistingessentially of atoms of transition metal, of magnesium and of halogen.The process is also suitable for use with Ziegler catalysts supported onsilica. The process is also especially suitable for use with metallocenecatalysts in view of the particular affinity and reactivity experiencedwith comonomers and hydrogen. The process can also be advantageouslyapplied with a late transition metal catalyst, i.e. a metal from GroupsVIIIb or Ib (Groups 8-11) of the Periodic Table. In particular themetals Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt are preferred, especiallyFe, Co and Ni. The late transition metal complex may comprise bidentateor tridentate ligands, preferably coordinated to the metal throughnitrogen atoms. As examples are those complexes disclosed in WO96/23010.Suitable iron and/or cobalt complexes catalysts can also be found inWO98/27124 or in WO99/12981.

[0107] It is also possible to use a high-activity catalyst consistingessentially of a chromium oxide activated by a beat treatment andassociated with a granular support based on a refractory oxide.

[0108] The catalyst may suitably be employed in the form of a prepolymerpowder prepared beforehand during a prepolymerisation stage with the aidof a catalyst as described above. The prepolymerisation may be carriedout by any suitable process, for example, polymerisation in a liquidhydrocarbon diluent or in the gas phase using a batch process, asemi-continuous process or a continuous process.

[0109] The following examples illustrate the present invention.

EXAMPLES

[0110] The following examples were conducted in a conventional fluidisedbed reactor. The catalyst used was a Ziegler type, titanium basedcatalyst (supported or pre-polymerised). The products made in theexamples were copolymers of ethylene and butene, and ethylene and4-methyl-pentene-1. Hydrogen was used as a chain transfer agent tocontrol the melt-index of the polymer.

[0111] The following examples are illustrations of the monitoring ofsheeting/agglomerate-free operating window. They correspond to a boundedwindow for instantaneous reacting particle properties. The mostsensitive parameter to adjust reacting particle properties for a givenproduct is operating temperature (final pellet Melt-Index and Densitybeing set for each product type).

[0112] The following examples will illustrate the use of operatingtemperature as a means to control reacting particle properties. Thefirst example is an illustration of operating conditions moving out ofthe optimised particle properties window. It is a comparative examplewhich illustrates the irreversible formation of sheets/agglomeratesthrough generation of (micro-)fines at the reactor wall when particleproperties are outside the optimum operating window.

[0113] The second example is an illustration of the optimum control ofparticle properties to avoid sheeting and agglomerates. This example isa product transition similar to the case of example 1. In this secondcase, temperature is adjusted to compensate for final resin propertychanges. This second example is an illustration of continuous operationin the sheeting/agglomerates-free operating window.

[0114] The third example is taken from WO99/02573. It is similar to thesecond example in terms of particle properties and final resinproperties. This example is an illustration of particle propertiesmoving outside the optimum window during grade transitioning. In thisexample, the meta-stable window has been chosen: the powder accumulationproblem is not solved but polymerisation temperature is decreased suchthat the layer of powder at the wall does not melt.

Comparative Example 1

[0115] Particle properties moving outside the optimum window duringgrade transitioning.

[0116] A fluidised bed reactor was transitioned from a 0.926 density,0.6 melt index ethylene/4-methyl-pentene-1 copolymer to a 0.935 density,0.5 melt index ethylene/4-methyl-pentene-1 copolymer. The prepolymer(Ziegler titanium based catalyst) was the same for both products. Thebed temperature was slightly decreased from 86° C. to 83° C. duringtransition to the higher density product.

[0117] The transition was smooth but as the 0.926 density, 0.6Melt-Index material was replaced by the 0.935, 0.9 Melt-Index resin,wall temperature started to peak in the lower part of the reactor as aconsequence of the formation of a fused layer of powder at the wall.Later on agglomerates started to block withdrawal lines.

[0118] In this case the tensile strength criteria is used to monitorinstantaneous reacting particle properties: the first product operatingconditions correspond to particle properties in the optimum operatingwindow (no sheeting nor agglomerates). During grade transitioning, thecriteria started to increase from 5.6 to 6.5 which is outside theoptimum window. The polymer instantaneously formed in the reactor becametoo brittle and fines and micro-fines started to form. Powder thenaccumulated at wall leading to overheating as it was observed on skintemperature probes and sheeting.

[0119] This typical problem of particle properties above the upper limitof the optimum window has been permanently solved by sufficientlyincreasing polymerisation temperature (in this case 95° C. so thecriteria equals 5.6) as it is illustrated in the following example.

Example 2

[0120] Particle properties are maintained in the optimum operatingwindow during grade transitioning.

[0121] A fluidised bed reactor was transitioned from a 0.919 density,0.9 melt index ethylene/butene copolymer to a 0.926 density, 0.75 meltindex ethylene/butene copolymer. The prepolymer (Ziegler titanium basedcatalyst) was the same as the one used in comparative example 1. The bedtemperature was increased from 86° C. to 96° C. during transition to thehigher density product with a rate such that the tensile strengthcriteria is maintained at 5.6.

[0122] Polymerisation temperature is increased to maintain particleproperties in the optimum window: not too close to sintering and not toobrittle/crystalline. For comparison, if polymerisation temperature hadbeen maintained at 86° C. during transition, the criteria would havereached 6.7 indicating that particle properties were far above the upperlimit of the optimum window (similar to example 1).

[0123] With such a transition procedure, particle properties remain inthe optimum window: no agglomerates/sheeting occurred and skintemperature probes remained at their baseline indicating that thereactor walls were clean.

Comparative Example 3

[0124] Particle properties are moving outside the optimum window duringgrade transitioning.

[0125] This example is taken from WO99/02573: the case is comparable tothe previous example which has been chosen for comparison.

[0126] A fluidised bed reactor was transitioned from a 0.917 density(instead of 0.919 for example 2), 0.6 melt index (instead of 0.9 forexample 2) ethylene/hexene copolymer to a 0.925 density (instead of0.926 for example 2), 0.5 melt index (instead of 0.75 for example 2)ethylene/hexene copolymer. The catalyst (Ziegler titanium-based) was thesame for both products. The bed temperature was increased from 86° C. to91° C. during the transition to the higher density product.

[0127] We have used the same tensile strength criteria to monitorinstantaneous particle property changes during the transition: the firstproduct is made at 86° C. which corresponds to a criteria of 5.5. Thisproduct is therefore in its optimum operating window thus explainingthat neither sheeting nor agglomerates have been experienced in thiscase. For the second product, the criteria reaches 6.2 which is outsidethe optimum window for particle properties. In fact, the value of 5.6would require us to operate at 97° C. (comparable to the similar casereported in the previous example). At 91° C., particle properties aretoo brittle and crystalline leading to the formation of a layer ofpowder at the wall. Unfortunately at 91° C., the temperature is highenough so the layer of powder can fuse and sheets start to form.Lowering operating temperature prevents the fusion of the layer but doesnot solve the problem of inadapted particle properties.

[0128] The change of particle surface properties is probably the reasonfor the increase of static level during transition: when the film startsto form, additional static is generated, and lowering operatingtemperature only stops this phenomena without solving the problem ofparticle properties: metastable operating conditions are reached withall the limitations we have described earlier: heat transfer capacity,and high sensitivity to operating parameters such as condensation,fluidisation velocity, polymerisation rate, and production rateregarding sheeting/agglomerates problem.

[0129] This last example is an excellent illustration of the use of theparticle properties criteria to monitor the sheeting and agglomeratesfree operating window. It underlines that the finding of this window isa breakthrough which was not obvious for the “Man of the Art” as itrequires to move counter to the prejudice of operating closer to powdersintering temperature. The criteria used to determine the optimumoperating window has proved to be extremely powerful as it alsodetermines the position of the optimum window not only for steady-stateoperation but at any time during transitions and start-ups as well.

Example 4

[0130] The catalyst used was2,6-diacetylpyridinebis(2,4,6-trimethylanil)FeCl₂ activated withmethylaluminoxane (MAO) and supported on silica (Crosfield grade ES70X).The preparation of this catalyst is described in detail in WO 99/46304,the content of which is incorporated herein by reference.

[0131] The polymerization was carried out in a conventional fluidizedbed gas phase polymerization reactor. The catalyst injection rate wasset such as to maintain the production rate constant at the desiredlevel. During the production of an ethylene polymer at a polymerizationtemperature of 90° C., cold bands on the reactor wall were observed; thepolymerization temperature was consequently increased to 96° C. and,within a short period of time, disappearance of cold bands could beobserved which is synonymous of having reached the optimum operatingwindow.

1. Process for reducing/suppressing sheeting or agglomerates duringtransition between two different polymer products made duringpolymerisation of olefins, characterised in that the instantaneousproperties of the growing polymer particles formed throughout thetransition are maintained such that there is no irreversible formationof agglomerates through generation of (micro-)fines.
 2. Processaccording to claim 1 wherein the instantaneous properties of the growingpolymer particles are predicted by using a structure/property model. 3.Process according to any of the preceding claims wherein mechanicalproperties are used as instantaneous properties of the growing polymer.4. Process according to claim 3 wherein tensile strength is used as theinstantaneous property of the growing polymer particles.
 5. Processaccording to any of the preceding claims wherein the instantaneousproperties of the growing polymer particles formed throughout thepolymerisation are maintained such that there is no irreversibleformation of agglomerates through generation of (micro-)fines throughcontrol of the operating temperature.
 6. Process according to any ofclaims 3 to 5 wherein mechanical properties are used as instantaneousproperties of the growing polymer.
 7. Process according to claim 6wherein tensile strength is used as the instantaneous property of thegrowing polymer particles.
 8. Process according to any of claims 3 to 7wherein the instantaneous properties of the growing polymer particlesformed throughout the polymerisation are maintained such that there isno irreversible formation of agglomerates through generation of(micro-)fines through control of the operating temperature.